The present invention relates to an image decoder which decodes a coded stream and an image transformation circuit which transforms an interlaced image into a progressive image in the decoding process.
For a stream encoded with standard resolution such as digital broadcast or DVD images to be decoded for audiovisual display on a high-definition TV monitor, it is necessary to convert the resolution (from interlaced images into progressive images) to improve the image quality. In the conventional progressive conversion method, interpolation scanning line signals generated by intra-field processing and interpolation scanning line signals generated by inter-field processing are combined according to image motion information to create interpolation scanning line signals.
In relation to the above method, JP-A-11-266440 describes a technique in which for an image signal series in an interlace scan, the mixture ratio of moving image interpolation scanning line signals generated by intra-field signal processing and still image interpolation scanning line signals generated by inter-field signal processing varies depending on the scalar amount of per-pixel motion vector to generate signals of scanning lines which are skipped in the interlace scan.
JP-A-2001-86508 discloses a technique in which an evaluation value is determined using a decoded motion vector and according to the evaluation value, intra-field interpolation signals and motion compensating interpolation signals are combined at a given mixture ratio and the combined signals are merged with decoded images before interpolation to output the resulting images.
JP-A-2004-336817 describes a technique in which motion compensating interpolation signals and moving image compensating signals are combined at the mixture ratio of motion compensation interpolation signals and intra-field interpolation signals which varies according to the absolute value of a motion compensation inter-frame differential signal, in order to generate interpolation scanning lines, where if the absolute value of the motion compensating inter-frame differential signal is below a threshold, motion compensation interpolation signals are generated by decoded motion vectors and if it is equal to or above the threshold, motion compensation interpolation signals are generated by zero motion vectors, and the moving image compensation signals are generated by intra-field signal processing.